Ring laser angular rate sensors, also called laser gyros, are well known in the art. One example of a ring laser angular rate sensor is U.S. Pat. No. 4,751,718 issued to Hanse, et al., which is incorporated herein by reference thereto. Present day ring laser angular rate sensors include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical closed-loop path.
Associated with such sensors is an undesirable phenomenon called lock-in which has been recognized for some time in the prior art. During lock-in the counter-propagating laser beams tend to lock together to a common frequency. To avoid or reduce the effects of lock-in, the ring laser gyro may be biased by an alternating bias technique such as that shown and described in U.S. Pat. No. 3,373,650 issued in the name of J. E. Killpatrick. The alternating bias technique is usually referred to as dithering, and may be implemented by a variety of ways including electro-optical and mechanical schemes. Since dithering, by either of these mentioned techniques and the like, directly affects the behavior of the counter-propagating laser beams, the sensor readout signal will contain not only inertial rate information, but will also contain a signal component directly related to the dithering, or alternating bias, of the sensor.
Dither motors of the prior art usually have a suspension system which includes, for example, an outer rim, a central hub member and a plurality of three dither motor reeds which project radially from the hub member and are connected between the hub member and the rim. Conventionally, a set of piezo-electric elements is connected to the suspension system. The set of piezo-electric elements serves as an actuator. When actuated through the application of an electrical signal to the piezo-electric elements, the suspension system operates as a dither motor which causes the block of the sensor to oscillate angularly at the natural mechanical resonant frequency of the suspension system. This dither motion is superimposed upon the inertial rotation of the sensor in inertial space. Such dither motors may be used in connection with a single laser gyro, or to dither multiple laser gyros. The prior art includes various approaches to recover inertial rotation data free from dither effects.
In situations where the gyro is mounted to an inertial platform or mounting base, and mechanical dithering is applied, the gyro output comprises the sum of the base angular motion and the gyro dither motion relative to the mounting base. This is true whether the sensor readout is mounted directly on the sensor, as in the case of a laser block mounted sensor readout, or fixed relative to the sensor mounting base like that shown in the aforementioned patent.
The signal contribution in the sensor readout signal due to the alternating bias is herein referred to as the dither signal component. For low noise navigational systems, the dither signal component in the readout signal generally needs to be minimized or removed to avoid control problems, particularly in block mounted readout systems.
Prior art solutions to remove the dither signal component include notch filters and fast fourier transforms. However, such techniques generate gain and phase shift disturbances which can affect the stability of any control loops or any readout signal processing systems. Another solution utilizes a digital pulse subtraction technique as taught in U.S. Pat. No. 4,248,534, issued to Elbert. A desirable solution is to remove the dither signal component by generating a correction signal which is substantially equivalent to the dither signal component. This latter approach is taught in U.S. Pat. No. 4,344,706 issued to Ljung et al. Ljung et al. teaches the use of a tracking circuit for tracking the clockwise and counterclockwise components of dither rotation. These dither components are subtracted from the usual readout signal which is responsive to the counter-propagating laser beams of the sensor thereby providing a corrected readout signal.
Another technique is to strip the dither signal component by an electronic dither compensator as taught in U.S. Pat. No. 4,610,543 issued to Ferriss. This patent illustrates an up/down count technique for a ring laser gyro readout mechanization in which readout output counts due to dither are subtracted from the readout output count in order to provide a corrected readout count.
The Ferriss patent also shows a closed-loop feedback method of reducing the dither component in the system readout signal. In Ferriss, a dither correction signal is subtracted from the gyro readout signal to derive a corrected sensor output signal. Closed loop correction is provided by generating the dither correction signal as a function of a dither reference signal representative of the dithering or alternating bias, and the relationship between the correction signal and the dither reference signal is controlled as a function of any dither signal component in the corrected sensor output signal.
Recently a modular Ring Laser Gyro (RLG) employing a microprocessor for control has been developed by Honeywell Inc. of Minneapolis, Minn., USA. Such a microprocessor controlled RLG requires highly precise stripped dither data to an accuracy of 1 to 2 counts from the readout of the modular RLG.
For a modular RLG it is advantageous that the dither stripping be done in response to external commands, that is, commands from outside of the modular RLG. If dither stripping is not done in response to external commands, the system frequency may be an exact sub-multiple of the dither frequency. In such a rare case, the dither stripper apparatus may not receive data allowing it to run accurately. Further, if the system frequency is close to a sub-multiple of the dither frequency, a very severe loss of gain may occur. Further still, loop gains, time constants, and other operational parameters are all dependent upon system request frequencies thus making design of an RLG having wide capabilities very difficult unless dither stripping commands are generated externally to avoid having timing which is close to system frequencies.